Rocks, resolution, and the record at the terrestrial K/T boundary, eastern Montana and western North Dakota

Reconstructions of mass extinction events are based upon faunal patterns, reconstructed from numerical and diversity data ultimately derived from rocks. It follows that geological complexity must not be subsumed in the desire to establish patterns. This is exemplified at the Terrestrial Cretaceous-Tertiary (K/T) boundary in eastern Montana and western North Dakota, where there are represented all of the major indicators of the terrestrial K/T transition: dinosaurian and non-dinosaurian vertebrate faunas, pollen, a megaflora, iridium, and shocked quartz. It is the patterns of these indicators that shape ideas about the terrestrial K/T transition. In eastern Montana and western North Dakota, the K/T transition is represented lithostratigraphically by the Cretaceous Hell Creek Formation, and the Tertiary Tullock Formation. Both of these are the result of aggrading, meandering, fluvial systems, a fact that has important consequences for interpretations of fossils they contain. Direct consequences of the fluvial depositional environments are: facies are lenticular, interfingering, and laterally discontinuous; the occurrence of fossils in the Hell Creek and Tullock formations is facies-dependent; and the K/T sequence in eastern Montana and western North Dakota is incomplete, as indicated by repetitive erosional contacts and soil successions. The significance for faunal patterns of lenticular facies, facies-dependent preservation, and incompleteness is discussed. A project attempting to reconstruct vertebrate evolution in a reproducible manner in Hell Creek-type sediments must be based upon a reliable scale of correlations, given the lenticular nature of the deposits, and a recognition of the fact that disparate facies are not comparable in terms of either numbers of preserved vertebrates or depositional rates

A New Crocodylian from the Late Maastrichtian of Spain: Implications for the Initial Radiation of Crocodyloids

The earliest crocodylians are known primarily from the Late Cretaceous of North America and Europe. The representatives of Gavialoidea and Alligatoroidea are known in the Late Cretaceous of both continents, yet the biogeographic origins of Crocodyloidea are poorly understood. Up to now, only one representative of this clade has been known from the Late Cretaceous, the basal crocodyloid Prodiplocynodon from the Maastrichtian of North America.The fossil studied is a skull collected from sandstones in the lower part of the Tremp Formation, in Chron C30n, dated at -67.6 to 65.5 Ma (late Maastrichtian), in Arén (Huesca, Spain). It is located in a continuous section that contains the K/P boundary, in which the dinosaur faunas closest to the K/P boundary in Europe have been described, including Arenysaurus ardevoli and Blasisaurus canudoi. Phylogenetic analysis places the new taxon, Arenysuchus gascabadiolorum, at the base of Crocodyloidea.The new taxon is the oldest crocodyloid representative in Eurasia. Crocodyloidea had previously only been known from the Palaeogene onwards in this part of Laurasia. Phylogenetically, Arenysuchus gascabadiolorum is situated at the base of the first radiation of crocodyloids that occurred in the late Maastrichtian, shedding light on this part of the cladogram. The presence of basal crocodyloids at the end of the Cretaceous both in North America and Europe provides new evidence of the faunal exchange via the Thulean Land Bridge during the Maastrichtian

The last dinosaurs of Brazil: The Bauru Group and its implications for the end-Cretaceous mass extinction

ABSTRACT The non-avian dinosaurs died out at the end of the Cretaceous, ~66 million years ago, after an asteroid impact. The prevailing hypothesis is that the effects of the impact suddenly killed the dinosaurs, but the poor fossil record of latest Cretaceous (Campanian-Maastrichtian) dinosaurs from outside Laurasia (and even more particularly, North America) makes it difficult to test specific extinction scenarios. Over the past few decades, a wealth of new discoveries from the Bauru Group of Brazil has revealed a unique window into the evolution of terminal Cretaceous dinosaurs from the southern continents. We review this record and demonstrate that there was a diversity of dinosaurs, of varying body sizes, diets, and ecological roles, that survived to the very end of the Cretaceous (Maastrichtian: 72-66 million years ago) in Brazil, including a core fauna of titanosaurian sauropods and abelisaurid and carcharodontosaurid theropods, along with a variety of small-to-mid-sized theropods. We argue that this pattern best fits the hypothesis that southern dinosaurs, like their northern counterparts, were still diversifying and occupying prominent roles in their ecosystems before the asteroid suddenly caused their extinction. However, this hypothesis remains to be tested with more refined paleontological and geochronological data, and we give suggestions for future work

Paleoenvironments of vertebrate-bearing strata during the Cretaceous-Paleogene transition, eastern Montana and western North Dakota ( USA).

Exposures of the Hell Creek and Tullock formations in eastern Montana and the Ludlow Formation in western North Dakota allow detailed reconstruction of the paleoenvironments associated with the Cretaceous-Paleogene (K-P) faunal transition in the Western Interior of North America. Facies associations demonstrate an ancient meandering fluvial environment in which gleization in unstable, poorly drained flood plains modified incipient soils. Sedimentologic and pedologic features indicate that concurrent with the faunal transition that occurred in the region, the amount of standing water increased dramatically, changing the earliest Paleogene soils and landscape. Depositional environment imposes taphonomic constraints on interpretations of K-P faunas and floras. Contrary to recent reports, fossil assemblages in Hell Creek channel deposits are reworked; bone and sediment clasts of the channel fills have been subject to traction transport. Age estimations based on supposedly unreworked fossils in channel deposits are thus unreliable. The chronostratigraphic resolution of the sediments under study is to date simply not comparable to the resolution required by researchers of the K-P boundary. - from Autho

Taphonomy and suggested structure of the dinosaurian assemblage of the Hell Creek Formation (Maastrichtian), eastern Montana and western North Dakota

This study quantifies the taphonomic context of fossil dinosaur elements in the Late Cretaceous Hell Creek Formation of Montana and North Dakota, USA. A previously published data base consisting of 649 individuals (counted at the family level) was used to establish statistically fundamental associations between the vertebrate faunal elements and the fluvial architectural elements in which they were found. In the Hell Creek, preservation is not equally distributed among the various fluvial architectural elements. Flood-plain and channel deposits preserve the preponderance of the Hell Creek dinosaur fauna. Articulated fossils most commonly occur within floodplain and point-bar deposits. Floodplain and related deposits, however, preserve the highest dinosaur faunal diversities. The dinosaur sample inferred to be most representative of the original dinosaurian assemblage structure, therefore, is obtained from floodplain and genetically related deposits. These yield eight families of dinosaurs represented in the following proportions: Ceratopsidae, 61%; Hadrosauridae, 23%; Ornithomimidae, 5%; Tyrannosauridae, 4%; Hypsilophodontidae, 3%; Dromaeosauridae, 2%; Pachycephalosauridae, 1%; and Troodontidae, 1%. Among these groups, dromaeosaurs and troodontids are represented only by teeth, a circumstance attributed at least in part to thin-walled bones whose potential for preservation in an active fluvial system is jeopardized. Ornithomimids constitute 5% of the total assemblage, which makes them the third most common dinosaur in this study. Their relatively high abundance may suggest a herbivorous dietary preference

Recognition and Interpretation of Cemented Subsurface Horizons in Sandy Paleosols of Cretaceous-Paleogene Age, Eastern Montana

Cemented paleosols of the Cretaceous Hell Creek Formation were examined in an attempt to discern origin of cementing agents. Six 3-m columnar sections were selected for detailed study. The soils were formed in interbedded sand and mud fluvial sediments and typically exhibit only weak signatures of pedogenesis. Furthermore, the sediments have been subject to low-temperature diagenesis that in some instances blurs the distinction between pedogenic and post-pedogenic features. In the sandstones, which are all interpreted as subsoil horizons, pedogenic features include oriented coatings of fine material and iron oxide coatings and hypo-coatings in intimate association with organic matter. In some horizons it appears that post-pedogenic phyllosilicates have replaced pedogenic coatings. Replaced grains and dense infillings of non-pedogenic voids by iron oxides were also identified. Calcite is evident in at least trace amounts in all sandy subsoils and is conspicuously abundant in medium textured horizons. However, the paleoenvironmental significance of calcite is difficult to assess because its abundance in any other context than as a framework constituent must be regarded at least in part as the result of repeated episodes of precipitation during diagenesis. © 1990

Paleosols spanning the Cretaceous-Paleogene transition, eastern Montana and western North Dakota ( USA).

Paleosols occur in exposures of the latest Cretaceous Hell Creek and Paleocene Tullock (=Ludlow) Formations in Montana and western North Dakota. The paleosols indicate that changes in ancient soil development occurred concomitantly with the better-known faunal transition. Features suggest that throughout the Cretaceous-Paleogene transition pedogenic processes in the region produced immature profiles, an observation consistent with the unstable, fluvial setting in which the ancient soils formed. Gleization was a dominant process in this setting, and podzolization modified some sandy soils. The assocation of the features enables recognition of O, A/E, Btg, Bhs, Bg, BC, Cg, and C horizons. During middle Hell Creek time, soils formed in a poorly drained setting that was only stable enough to permit incipient pedogenesis. The bulk of the pedogenesis occurred in levee and flood-plain deposits; soils also occurred on point-bar and crevasse-splay deposits. In topographically depressed regions, organic accumulations formed with minimal soil development. Matted plant debris is the product of this environment. Gley features and segregations of iron oxides around voids suggest fluctuation of the water table. By latest Hell Creek time, the mean level of the water table rose, and in the lowest Tullock (Ludlow) exposures, extensive ponded deposits are preserved. Vegetation accumulated at a rate sufficient for coal formation. The amount of fluctuation apparently was reduced, and pedogenesis was further inhibited, as indicated by the virtual absence of illuviated clays in the sediments.-from Author

Major extinctions of land-dwelling vertebrates at the Cretaceous- Tertiary boundary, eastern Montana

A large database recording species of terrestrial vertebrates present in formations above and below the Cretaceous-Tertiary (K-T) boundary in eastern Montana was assembled by J.D.Archibald and L.J.Bryant. Division of the species in this database into freshwater and land-dwelling vertebrate assemblages reveals that the K-T vertebrate extinction was concentrated in land-dwelling forms. In data corrected for the effects of rare taxa, 90% of the species in the freshwater assemblage survived into the T, but only 12% of the land-dwelling forms survived. The pattern of differential extinction of terrestrial vertebrates in eastern Montana may be in large part the result of the dependence of land-based communities on primary productivity. This is in contrast to the riverine communities, which may derive much of their organic carbon from detritus. The pattern of extinction and survival is compatible with the hypothesis of an asteroid impact after which there was a temporary cessation of primary, photosynthetic productivity. -Author

Paleocene paleosols of the petrified forests of Theodore Roosevelt National Park, North Dakota: a natural experiment in compound pedogenesis

The Petrified Forest Plateau of Theodore Roosevelt National Park (TRNP) in western North Dakota provides an excellent setting for exploring the influence of ancient, exhumed land surfaces upon modern ones. Water was abundant in the Plateau region, and during much of the time the landscapes were submerged. At least twice, large forests developed in soils forming on floodplain sediments. While it is clear that soils do respond to environments, it is also clear that the response may vary, depending upon the nature of the soil material and morphologies inherited. Here, the development of a gleyed morphology has proven largely irreversible. -from Author